An interfering signal at the antenna of a traditional closed loop polar modulation system having a frequency synthesizer can cause the frequency synthesizer to attempt to lock on to the interfering signal. A frequency synthesizer lock on to an interfering signal will usually result in a complete failure of the communication functions of a device incorporating the traditional closed loop polar modulation system. Open loop polar modulation systems have been developed to reduce the possibility of a frequency synthesizer locking on to interfering signals. At worst, these open loop polar modulation systems will, suffer an error for one timeslot before a correction is made.
In an open loop polar modulation system, correction tables for amplitude modulation to amplitude modulation (AMAM) distortion and amplitude modulation to phase modulation (AMPM) distortion are developed during a one-time factory calibration of the open loop polar modulation system. As a result of the one-time factory calibration, the correction tables are permanently fixed for the life of a device such as a mobile terminal that incorporates the open loop polar modulation system. A weakness of such traditionally factory calibrated open loop polar modulation systems is that the correction tables for the AMAM distortion or the AMPM distortion will not be accurate under changing conditions like temperature and voltage standing wave ratio (VSWR). Consequently, there will be degradation of performance for error vector magnitude (EVM) and spectral purity.
Open loop polar modulation systems may overcome this weakness through the use of an adaptive feedback system in which the correction tables for the AMAM distortion and the AMPM distortion are updated during a ramp up of each transmission burst. Thus, changes in phase and amplitude of radio frequency (RF) output from a power amplifier (PA) in communication with the adaptive feedback system are corrected in a dynamic fashion. However, a new problem is introduced due to the adaptive nature of traditional open loop polar modulation systems. Specifically, an amplitude and phase feedback for driving open loop polar modulation systems must be largely linear with little AMPM distortion, otherwise the correction tables for AMAM distortion and AMPM distortion will become corrupted. The corruption of the correction tables will result in miscorrection of the AMAM distortion and the AMPM distortion, which in turn will result in erroneous feedback being passed on to the PA through PA feedback networks. What is needed is polar feedback linearization that provides amplitude and phase feedback that is largely linear, with little AMPM distortion, for driving open loop polar modulation systems.